1. Field of the Invention
The present invention relates to a shape measuring apparatus.
2. Description of the Related Art
Shape measuring apparatuses are known which are equipped with a probe having a stylus tip for measurement of a workpiece to be measured, a movable mechanism for moving the probe, and a control device for controlling the movable mechanism and which measure the shape of a workpiece to be measured by moving the stylus tip so that it scans the surface of the workpiece to be measured (refer to JP-A-2008-241420, for example).
In the apparatus disclosed in JP-A-2008-241420, movement velocities of a probe are calculated by converting design values (NURBS (non-uniform rational B-spline) data) which are based on CAD data or the like into PCCs (parametric cubic curves) and calculating velocity curves from divisional PCCs obtained by dividing the PCCs. The probe is moved based on the movement velocities calculated based on the design values, whereby a stylus tip is moved so as to scan the surface of a workpiece to be measured (nominal scanning: scanning known paths generated based on the design values).
Shape measuring apparatuses of another type are known in which design value data are not used. Instead, an amount of the push of a probe toward a workpiece to be measured is detected and the probe is moved so as to scan the surface of the workpiece to be measured so that the probe deflection becomes equal to a prescribed reference value (autonomous scanning: scanning unknown paths).
Incidentally, the nominal scanning as described in JP-A-2008-241420 enables a high-speed shape measurement because a stylus tip is moved along a path that is set according to design values. However, no consideration is given to a variation of the amount of the push of a probe toward a workpiece to be measured. Therefore, for example, when a stylus tip arrives at a recess that should not exist, in a design, in the surface of a workpiece to be measured, an event may generate that the deflection decreases and the stylus tip separates from the surface of the workpiece to be measured. This means a problem that the measurement accuracy is lowered.
On the other hand, in the autonomous scanning, since a probe is moved so that the deflection is kept constant, the probability that a stylus tip separates from the surface of a workpiece to be measured and hence a high-accuracy shape measurement is enabled. However, to accommodate, for example, workpieces to be measured having rapidly varying radii of curvature, it is necessary to set the probe movement speed low. This means a problem of a long measurement time.